Different from permanent hair dye compositions containing oxidative dye intermediates, direct hair dye compositions containing anionic or cationic direct dyes do not require a coupling reaction with an oxidizing agent. They are hence widely applied to hair in the form of so-called color shampoos along with surfactants or in the form of concentrates such as lotions, emulsions or gels. In addition, proposals have also been made on clear hair dyes containing so-called cationic dyes of a construction that a cationic group is contained in a conjugated system.
These direct hair dye compositions are accompanied by a drawback that their colors are vivid shortly after dyeing but substantially fade out with time Moreover, when mixed with an alkaline peroxide commonly employed as an oxidizing agent, these direct dyes are readily decomposed and may not be able to bring about their dyeing effects as expected They, accordingly, involve another drawback that, when employed as hair dyes, such hair dyes are limited to hair dye compositions having no bleaching effects on hair. In addition, they are unstable to alkalizing agents and reducing agents which are essential ingredients for so-called permanent hair dyes containing oxidative dyes, leading to a further drawback that they cannot be used together with such alkalizing agents and reducing agents in permanent hair dyes.
On the other hand, 2,1,3-benzothiadiazole derivatives are compounds known as a kind of azo dyes (see, for example, Patent Document 1, Patent Document 2 and Non-patent Document 1). They are, however all dissatisfactory for use as hair dye compositions from the viewpoints of color hue, fastness and molecular extinction coefficient.
Patent Document 1: GB-A-1550620
Patent Document 2: IT-A-778433
Non-patent Document 1: Bollettino Scientifico della Facolta di Chimica Industriale di Bologna, 26(3-4), 215-216 (1968).